Identification of Bacterial Fish Pathogens in Brazil by Direct Colony PCR and 16S Rrna Gene Sequencing

Advances in Microbiology, 2015, 5, 409-424 Published Online June 2015 in SciRes. http://www.scirp.org/journal/aim http://dx.doi.org/10.4236/aim.2015.56042 Identification of Bacterial Fish Pathogens in Brazil by Direct Colony PCR and 16S rRNA Gene Sequencing F. A. Sebastião, L. R. Furlan, D. T. Hashimoto, F. Pilarski* Aquaculture Center of São Paulo State University—CAUNESP, Universidade Estadual Paulista, Jaboticabal, Brazil Email: *[email protected] Received 14 May 2015; accepted 11 June 2015; published 16 June 2015 Copyright © 2015 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution-NonCommercial International License (CC BY-NC). http://creativecommons.org/licenses/by-nc/4.0/ Abstract Intensive fish farming systems in Brazil have increased the disease incidence, mainly of bacterial origin, due to higher stocking density, high organic matter levels and poor quality of the aquatic environment that causes high mortality rates during outbreaks. The identification of pathogenic species using a fast and reliable method of diagnosis is essential for successful epidemiological studies and disease control. The present study evaluated the use of direct colony PCR in combina- tion with 16S rRNA gene sequencing to diagnose fish bacterial diseases, with the goal of reducing the costs and time necessary for bacterial identification. The method was successful for all 178 isolates tested and produced bands with the same intensity as the standard PCR performed using pure DNA. In conclusion, the genetics methods allowed detecting the most common and important pathogens in Aquaculture, including 12 species of occurrence in Brazilian fish farms. The results of the present study constitute an advance in the available diagnostic methods for bacterial pathogens in fish farms. Keywords Direct Colony PCR, 16S rRNA Sequencing, Bacterial Fish Pathogens 1. Introduction Due to its high water availability and favorable climate conditions, Brazil displays high potential for the devel- opment of fish farming, which is an activity that has been growing substantially over the last few years. Ac- *Corresponding author. How to cite this paper: Sebastião, F.A., Furlan, L.R., Hashimoto, D.T. and Pilarski, F. (2015) Identification of Bacterial Fish Pathogens in Brazil by Direct Colony PCR and 16S rRNA Gene Sequencing. Advances in Microbiology, 5, 409-424. http://dx.doi.org/10.4236/aim.2015.56042 F. A. Sebastião et al. cording to the Food and Agriculture Organization of the United Nations [1], Brazil is the second largest aquaculture producer in Latin America and the Caribbean, and freshwater aquaculture (tilapia, carp, and native fish) represented 87% (545,300 ton) of the total aquaculture production in 2011. The growing interest in this activity and, consequently, the search for higher profitability, have been leading producers to adopt super-intensive production systems. However, the high density of confined fish, inadequate farming management practices, and water contamination by toxic products cause chronic stress and immuno- suppression in farmed animals. These effects lead to the occurrence of diseases and epizootic outbreaks caused by pathogens that would not have high expression in natural environments [2]. Knowledge about the etiological agents, pathogenesis, biochemistry, antigenicity, epizootiology, and inter-relationship of stress and environmen- tal factors of bacterial infections affecting fish is essential to avoid and control diseases. However, these factors have not been well studied, especially because fish farming is a recent activity, with its intensification beginning in the 1990s [3]. Gram-negative bacteria such as Aeromonas, Flavobacterium, Pseudomonas, and Francisella and gram-positive bacteria from the genera Streptococcus and Lactococcus [4]-[11] are some of the pathogens responsible for economic losses in Brazil. They can cause high fish mortality rates up to 72 h after infection [12]. Although the number of studies focusing on the diagnosis of bacterial etiological agents has increased over the last few years, there are still few available alternatives for the control of fish bacterial infections in Brazil. Therefore, quicker and more effective diagnostic alternatives are necessary, which would help control diseases before they lead to irreversible clinical consequences and high mortality rates. Molecular diagnostic methods use reduced volumes of sample material and exhibit high sensitivity, specificity, and accuracy in pathogen detection [13]. Methods that do not require purified DNA extraction, such as direct colony PCR, are quicker and less expensive and may greatly aid in the early detection of fish pathogens [14]. In addition, because not all microorganism sequences are catalogued in current databases, the use of universal and degenerate primers is a wise strategy. For this reason, methods based on 16S rRNA ribosomal gene amplification and sequencing have been widely ex- plored [13]. The use of universal PCR primers is based on the hypothesis that the primers used are complementary to con- served regions of genes in the environment, resulting in amplification; in turn, heterogeneity is found inside of the fragments flanked by the primers, in hypervariable regions [15] [16]. This method has been revolutionizing microbial ecology, from studies of non-cultivable bacteria to the correct identification of pathogens for accurate diagnoses. The aim of the present study was to evaluate the direct colony PCR combined with 16S rRNA gene sequencing as a faster and less expensive method to identify fish bacterial pathogens, compared to the classic PCR pro- tocol. Moreover, we have used these methods to demonstrate the efficiency of genetic approaches for the prac- tical evaluation of the diagnosis of aquaculture diseases in Brazilian fish farms. 2. Materials and Methods 2.1. Bacterial Strains and Culture Conditions 178 bacterial isolates were obtained between 2010 and 2014 from the following hosts (n = number of assessed fish): tilapia (Oreochromis niloticus n = 93), tambaqui (Colossoma macropomum n = 10), carp (Cyprinus carpio n = 3), cachara (Pseudoplatystoma reticulatum n = 34), and pacu (Piaractus mesopotamicus n = 8). The hosts exhibited clinical signs of bacterial diseases, such as skin ulcerative lesions, hemorrhagic septice- mia, meningoencephalitis, fin rot, exophthalmia, and were collected at fish farms in different regions of Bra- zil:Dourados (Mato Grosso do Sul State, 22˚13'16"S, 54˚48'20"W, n = 38), Rio de Janeiro (Rio de Janeiro State, 22˚54'S, 43˚10'W, n = 4), Itambaracá (Paraná State, 23˚0'49"S, 50˚24'7"W, n = 10), Itaju (22˚25'37''S, 45˚27'11''W), Arealva (22˚1'38''S, 48˚54'36''W), Porto Ferreira (21˚51’18’’S, 47˚28'45''W), Guaíra (20˚19'5''S, 48˚18'42''W), Santa Fé do Sul (20˚12'43''S, 50˚55'38''W), Palmital (22˚47'30''S, 50˚12'18''W) and Jaboticabal (21˚15'19''S, 48˚19'21''W—São Paulo State, n = 123). For the isolation of bacteria, scrapings were performed using sterile swabs on fish kidneys and brain. Gram- negative colonies were plated on TSA (Tryptic Soy Agar-Biolife), and TSB (Tryptic Soy Broth-Biolife) and incubated for 24 h in bacteriological incubator adjusted to 28˚C. While gram-positive colonies were seeded in Columbia blood agar (Difco) incubated for 24 - 72 h at 30˚C and subcultured in BHI (Brain Heart Infusion 410 F. A. Sebastião et al. Broth, Himedia). The strains of Palmital (SP) were obtained directly from the Laboratory of Aquatic Animal Disease, APTA, Votuporanga, SP. 2.2. Molecular Identification of Isolates Two methods of molecular diagnosis were compared in this study aiming to evaluate the efficiency of direct colony PCR (time and cost effectiveness) in relation to the PCR amplification of purified DNA by extraction, both combined with gene sequencing (Table 1). The standard PCR of purified DNA method followed the steps below. 2.2.1. DNA Extraction One colony of each isolated was transferred to a tube containing appropriate liquid culture medium (TSB for gram-negative and BHI for gram-positive) and incubated at 28˚C until the OD600 was between 1 and 1.5. Fol- lowing incubation, 1.0 mL of the bacteria culture was centrifuged at 12,000×g for 1 min, the supernatant was discarded, and the pellet was frozen at −20˚C until DNA extraction. The Axyprep® miniprep kit for bacterial genomic DNA was used according to the manufacturer’s instructions (Axygen Biosciences, Union City, CA, USA). DNA was quantified by fluorometry using a Qubit 2.0 fluorometer (Life Technologies, NY, USA). 2.2.2. Standard PCR PCR was performed in a 25 µL final volume, containing 2.5 µL of 10X buffer (10 mM Tris-HCl, 50 mM KCl), 0.2 µL of 25 mM dNTP, 1.0 µL of 50 mM MgSO4, 0.2 µL of Taq High Fidelity (Platinum®Taq DNA Polyme- rase, Life Technologies, NY, USA), 2.0 µL of each primer (10 pmol), 25 ng of DNA template, and Milli-Q water up to the final volume. The PCR program consisted of 94˚C for 2 min; 35 cycles of 94˚C for 30 seconds, 55˚C for 30 s, and 68˚C for 1.5 min; and final extension at 68˚C for 10 min. We used the primers 8F/907R (Table 2), specific for the 16S rRNA bacterial gene [15] [17] [18]. The resulting amplicons of approximately 900 bp (base pair) were analyzed by electrophoresis in 1.5% agarose gel stained with ethidium bromide, according to Sambrook et at. [19]. 2.2.3. Purification of PCR Products and Gene Sequencing PCR products were purified using a MinElute Kit (Qiagen, Crawley, West Sussex, UK) according to the manufacturer’s instructions. Purified PCR products were quantified using a Qubit 2.0 fluorometer, and gene sequencing was performed using 50 ng/µL per sample. Sequencing was performed according to Sanger [20]. PCR products were amplified using AmpliTaq polymerase and BigDye Terminator (Applied Biosystems) according to the manufacturer’s instructions, using the primer 907R.

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